The present patent application claims the benefit of earlier Japanese Patent Application No. H11-237956 filed Aug. 25, 1999, the disclosure of which is entirely incorporated herein by reference.
1. Field of the Invention
This invention relates to a power circuit breaker, which can instantaneously interrupt a power source circuit by disconnecting a fuse element from the circuit making use of a gas pressure of an igniter.
2. Description of the Related Art
FIG. 1 illustrates a conventional fuse 61 used to break a high-current circuit. The fuse 61 includes a housing 62 made of a synthetic resin, a fuse element 63 made of a conductive metal and accommodated in the housing 62, and a cover 64 for capping the top opening of the housing 62.
The fuse element 63 comprises a base body 65 bent into an inverse U-shape, a pair of female terminal pieces 66 extending from both ends of the base body 65, and a tin chip (i.e., a heat storage) 67 placed on the top face of the base body 65. Each female terminal piece 66 is combined with an elastic contact piece 68, which is positioned separate from the baseboard 63. The female terminal piece 66 and the elastic contact piece 68 constitute a female terminal to receive a male terminal of a fuse box or the like (not shown). The male terminal is inserted from the lower opening 69, and it comes into contact with the elastic contact piece 68 of the female terminal.
The base body 65 also has a pair of stoppers 70 that are formed monolithically with the base body 65. Each stopper 70 catches the shoulder of the inner wall of the housing 62, thereby preventing the fuse element 63 from coming off the housing 62. The base body 65 of the fuse element 63 blows out if an excessive amount of electric current flows through it. The blowout of the fuse element 63 causes the power source circuit to be cut off.
FIG. 2 is a graph showing the breaking characteristics of the conventional fuse 61 shown in FIG. 1. The horizontal axis denotes an electric current, and the vertical axis denotes a blowout time T, which is indicated in a logarithmic scale.
As an electric current I flowing through the fuse 61 increases, the blowout time T of the fuse 61 decreases along a quadratic curve. The blowout time T becomes very long at a lower range of excessive current.
In the example shown in FIG. 2, the fuse 61 is designed so that the electric current through it in the normal use is 60 Amp, which are about 50% of the rated current. If the excessive current is small, that is, if an electric current of 70 Amp or 80 Amp flows through the fuse 61, then the fuse 61 does not blow out for a long time, as indicated by the circle A in FIG. 2, even through some faults occur in the circuit.
This means that it is difficult for the conventional fuse 61 to instantaneously break the circuit when the quantity of excessive current is small because the fuse element does not blow out immediately. The same defect applies to a situation in which an intermittent short circuit, such as a rare short, occurs. As still anther situation, if a short circuit has occurred in a load circuit, the temperature of the fuse element 63 does not rise up to the blowout temperature in spite of the overcurrent. This also prevents the fuse element 63 from blow out promptly.
To overcome these problems, a power circuit breaker 76 shown in FIG. 3 was proposed. The prior art power circuit breaker 76 electrically senses an overcurrent, and shuts off the circuit forcibly by means of an ignition pressure of the igniter.
The power circuit breaker 76 has a pair of terminals 77 and 78, each of which has a multi-contact-point spring 80. A conductive shaft 79 is in contact with the multi-contact-point springs 80 in a slidable manner. An igniter 81 is placed behind one of the multi-contact-point spring 80 of the terminal 78.
The base of the shaft 79 is secured to an operation shaft 82, which is furnished with a torsion spring 83. The igniter 81 is filled with a gas-blasting agent, and a heater is placed inside it. The heater is connected to a lead 84. Both the shaft 79 and the operation shaft 82 are placed in the housing 85 in a slidable manner.
The terminals 77 and 78 are electrically connected with the shaft 79 via the multi-contact-point springs 80. If an excessive current flows through the terminals 77 and 78, the sensor (not shown) senses the change, and causes an electric current to flow through the lead 84 to the heater. The heater heats the gas-blasting agent, and the shaft 79 is pushed toward the disconnected position under a gas pressure of the igniter, as shown in FIG. 2B. The electric conductivity between the terminals 77 and 78 are now cut off. The shaft 79 is prevented from returning to the original position because the torsion spring 83 forces a stopper 86 to project outward and catch the edge of the housing 85.
However, a problem in the power circuit breaker 76 is that the igniter 81 is not be activated when the quantity of excessive current is below the minimum sensible current of the sensor, as in the conventional fuse 61 shown in FIG. 1. This situation often occurs, for example, if a rare short happens, a short circuit occurs in the load circuit, or no excessive current flows through the terminals 77 and 78. In these cases, the power source circuit can not be appropriately interrupted.
The present invention was conceived to overcome these problems in the prior art, and it is an object of the invention to provide a power circuit breaker that can break the power source circuit without fail even under a small amount of excessive current. The power circuit breaker also reliably works if a short circuit occurs in the load circuit.
In order to achieve the object, a power circuit breaker according to the invention has a housing, a fuse element placed in the housing, and a temperature sensitive fuse attached to the fuse element. The power circuit breaker also has switching means connected to the temperature sensitive fuse, and an igniter connected to the switching means. The fuse element has a pair of tab terminals, which are received in trunk terminals connected to an external circuit.
Preferably, the switching means is a transistor. The igniter is filled with a gas-blasting agent, and a heater is placed in the gas-blasting agent. The transistor is connected to the heater.
If an excessive current flows through the fuse element due to a short circuit having occurred in a load circuit, the temperature of the fuse element rises, but still below the blowout temperature of the fuse element. The temperature sensitive fuse attached to this fuse element is sensitive to a change in the temperature of the fuse element, and it blows out in response to a small rise of the temperature. The blowout of the temperature sensitive fuse immediately causes the transistor to turn on, and the heater is electrically connected. The gas-blasting agent is heated, and a gas pressure causes the tab terminals of the fuse element to be detached from the trunk terminal, whereby the power source circuit is cut off.
Preferably, a circuit board, on which the switching means is incorporated, is accommodated in the housing. The melted temperature sensitive fuse is easily replaced with a new one.